use attoparsec parser for String parsing, 10x speedup

This is not as efficient as using ByteStrings throughout, but converting
the String to ByteString is actually significantly faster than the old
parser.

    benchmarking parse/old
    time                 9.657 μs   (9.600 μs .. 9.732 μs)
                         1.000 R²   (0.999 R² .. 1.000 R²)
    mean                 9.703 μs   (9.645 μs .. 9.785 μs)
    std dev              231.6 ns   (161.5 ns .. 323.7 ns)
    variance introduced by outliers: 25% (moderately inflated)

    benchmarking parse/new
    time                 834.6 ns   (797.1 ns .. 886.9 ns)
                         0.987 R²   (0.976 R² .. 0.999 R²)
    mean                 816.4 ns   (802.7 ns .. 845.1 ns)
    std dev              62.39 ns   (37.66 ns .. 108.4 ns)
    variance introduced by outliers: 82% (severely inflated)

There is a small behavior change from the old parsePOSIXTime,
which accepted any amount of trailing whitespace after the timestamp.
That behavior was not documented, and it doesn't seem anything relied on it.

Utility/InodeCache.hs

@@ -156,7 +156,7 @@ readInodeCache s = case words s of
 	(inode:size:mtime:mtimedecimal:_) -> do
 		i <- readish inode
 		sz <- readish size
-		t <- parsePOSIXTime' mtime mtimedecimal
+		t <- parsePOSIXTime $ mtime ++ '.' : mtimedecimal
 		return $ InodeCache $ InodeCachePrim i sz (MTimeHighRes t)
 	_ -> Nothing